All multicellular organisms originate from a small set of totipotent embryonic stem cells which expand and differentiate into tissues and organs of a mature individual. However, individuals are not static entities, and during normal growth and aging, or after injury, differentiated cells, tissues and organs must be replenished and regenerated. In adults, this process of regeneration is thought to be carried out by stem or progenitor cells which retain the capacity to expand and differentiate throughout the lifespan of the individual. However, the nature of these cells and the cellular and molecular mechanisms which control their expansion and differentiation are not well-understood, but have important clinical significance. Using model systemsthat provide experimentally accessible and reliable regenerative potential can facilitate our understanding of the biology underlying regeneration. The overall objectives of this proposal are designed to isolate and characterize stem cells from the primitive chordate, Botryllus schlosseri. B. schlosseri provides a unique model to study stem cell biology for two reasons. First, regeneration is a major part of the life history of Botryllus: in a highly coordinated developmental process, Botryllus adults regenerate themselves, including all somatic tissues and the germline, every week. Secondly, under natural conditions the cells responsible for this regeneration can mobilize and transplant between two individuals. Once transplanted, these cells can proliferate and out compete host stem cells and take over the other individual independently at the level of the germline and in the soma for the remaining lifetime of the host, in processes called germline cell parasitism (gcp) or somatic cell parasitism (scp), respectively, and we have shown that there is a genetic basis to this process. Fundamental, conserved and important aspects of stem cell biology, such as self- renewal capacity, homing or expansion and differentiation kinetics must underlie the ability of a stem cell of one genotype to out compete a stem cell of another genotype. The overall goals of this proposal are to use this system to prospectively isolate the cell(s) responsible for gcp and scp, determine the cellular and molecular biological phenotype of these cell(s), and correlate those phenotypes with parasitic capability.